In general, desalination and purification of saline or polluted water using buoyant gas hydrates is known in the art. For example, U.S. Pat. No. 5,873,262 discloses a water desalination or purification method wherein a gas or mixture of gases spontaneously forms buoyant gas hydrate when mixed with water at sufficiently high depth-induced pressures and/or sufficiently low temperatures in a treatment column. According to prior technology, the treatment column is located at sea. Because the hydrate is positively buoyant, it rises through the column into warmer water and lower pressures. As the hydrate rises, it becomes unstable and dissociates into pure water and the positively buoyant hydrate-forming gas or gas mixture. The purified water is then extracted and the gas is processed and reused for subsequent cycles of hydrate formation. Suitable gases include, among others, methane, ethane, propane, butane, and mixtures thereof.
Methods of desalination or purification using buoyant gas hydrates known prior to Applicant's co-pending application Ser. No. 09/350,906 rely on the naturally high pressures and naturally low temperatures that are found in open ocean depths below 450 to 500 meters when using pure methane (or at shallower depths when using mixed gases to enlarge the hydrate stability “envelope”), and the desalination installations, being fixed to pipelines carrying fresh water to land, are essentially immobile once constructed. In certain marine locations such as the Mediterranean Sea, however, the water is not cold enough for the requisite pressure to be found at a shallow enough depth; this would necessitate using a much longer column, which may be impractical.
In addition to the temperature of the seawater, other heat considerations are relevant to systems for desalination or purification of water using gas hydrates. When gas hydrate forms, it gives off heat in an exothermic reaction due to significantly higher heats of fusion than water-ice. In a hydrate fractionation desalination apparatus, the cold water and high pressures required for natural hydrate formation in the sea are reproduced within the desalination apparatus. According to this approach to water desalination or purification, in order for a gas or mixture of gases spontaneously to form gas hydrate when mixed with treatment water at sufficiently high pressures, the treatment water must be of sufficiently low temperature.
Because the stability of hydrate is governed by both the temperature and pressure of the water-to-be-treated in which the hydrate forms, in certain circumstances, only a certain amount of hydrate can be formed before the heat generated by the exothermic formation of hydrate raises the temperature of the residual water to a level at which hydrate will no longer form. In other words, for a given volume of water-to-be-treated, which volume of water initially is suitable for the spontaneous formation of hydrate when hydrate-forming gas is introduced, the formation of hydrate itself can limit the amount of hydrate which will form because of the associated rise in temperature. Accordingly, there is a need for methods and systems that overcome this effective self-limitation.
Furthermore, laboratory and at-sea experimental experience has shown that causing gas hydrate to form spontaneously in seawater of suitable pressure and temperature can result in different forms of hydrate where the hydrate formation is carried out under different physical chemical conditions. For example, when the hydrate-forming gas is introduced to open ocean seawater that is very undersaturated in hydrate-forming-gas, hydrate formation is generally restricted to a zone at the interface between water and gas resulting in the formation of relatively thin aggregates of hydrate. Hydrate formation under these conditions is less efficient and results in the production of a very large number of small pieces of hydrate which can have shapes which are not as hydrodynamic as larger pieces of solid hydrate. The small pieces of hydrate that are not hydrodynamic rise buoyantly at slower rates than larger pieces of solid hydrate, and thus begin (and likely complete) their dissociation at depths deeper than is desired for optimal conversion of hydrate to water and gas.